1. Field of the Invention
The present invention relates to a failure diagnostic apparatus for atmospheric pressure sensors used in an engine control system. More specifically, the present invention relates to a failure diagnostic method and apparatus for atmospheric pressure sensors in a system having a first atmospheric pressure sensor and a second atmospheric pressure sensor that backs up the first atmospheric pressure sensor.
2. Description of Background Art
Hitherto, a number of sensors have been used in an engine control system and a failure diagnostic system for diagnosing whether or not these sensors are normal is known. For example, JP-A-2003-307152 discloses a method including a step of determining whether or not an output signal of a sensor that senses pressure in an intake manifold is within a preset range, a step of calculating the pressure in the intake manifold on the basis of engine operating conditions, and a step of determining whether or not a difference between the pressure in the intake manifold indicated by the output signal from the sensor and the calculated pressure in the intake manifold is smaller than a set value when the output signal from the sensor is within the range, wherein when the output signal from the sensor is not within the range or when the difference is not smaller than the set value, the calculated pressure is set as the pressure in the intake manifold.
In an electronically controlled throttle apparatus disclosed in JP-A-10-176582, two systems each including an accelerator sensor, a throttle sensor, and so on are provided as a failsafe device or the like. Failure diagnosis is performed by comparing sensor outputs of the two systems. For example, by comparing a deviation between a main throttle sensor and a sub throttle sensor and a predetermined threshold value, it is determined whether or not the throttle sensor has failed.
Failures such as a disconnection and a short circuit of the sensor system can be determined when a sensor output value is too high or too low with respect to a predetermined value or a predetermined range. These types of failures can be addressed in a manner that is determined in advance. However, in some situations, a change or a deterioration of output characteristics, which may impair normal control even though the sensor output value is within the predetermined range, cannot be determined as a failure.
On the other hand, the diagnostic method disclosed in JP-A-10-176582 can detect that either one of the two system sensors has failed. However, it is difficult to determine which one of the two system sensors has failed.